Electroluminescent panel which is equipped with light extraction elements

ABSTRACT

An image-display panel is described. The panel includes a one-dimensional or two-dimensional matrix of organic electroluminescent cells deposited on a substrate and grouped together in rows or columns. Light extraction elements are deposited on each row or column of cells to form an extraction layer.

This application claims the benefit, under 35 U.S.C. §365 ofInternational Application PCT/FR03/00278, filed Jan. 30, 2003, which waspublished in accordance with PCT Article 21(2) on Sep. 4, 2003 in Frenchand which claims the benefit of French patent application No. 0202476,filed Feb. 27, 2002.

The invention relates to an illumination or image-display panelcomprising a one-dimensional or two-dimensional matrix of organiclight-emitting cells (or OLEDs) which is provided with means for makingit easier to extract the light emitted by these cells, which meansconsiderably improve the luminous efficiency thereof.

Such a panel generally comprises a substrate that supports a thinorganic electroluminescent layer inserted between two arrays ofelectrodes, one being an array of anodes and the other an array ofcathodes, which are intended to supply the cells; each cell ispositioned in a region of overlap between an anode and a cathode; in thecase of a passive-matrix panel, each array is generally formed fromelectrodes in the form of parallel bands of constant width; theelectrodes of the anode array are generally perpendicular to theelectrodes of the cathode array; for polychromatic, especiallytrichromatic, panels, the thin organic electroluminescent layer isgenerally divided into alternating bands of different emission colors.

In the case of active-matrix panels, the substrate incorporateselectronic components for driving the cells, in the case of apassive-matrix panel, the substrate is generally made of glass orplastic; the thickness of the substrate is generally between 300 μm and1500 μm, i.e. 500 to 100 times greater than that of the cells; thedimension or the diameter of the cells or pixels is generally between100 μm and 300 μm, i.e. 1 to 15 times smaller than the thickness of thesubstrate; the layer of electrodes inserted between the substrate andthe electroluminescent layer is generally called the “lower layer”since, in conventional fabrication processes, it is applied before theelectroluminescent layer; the other layer of electrodes, which isapplied after the electroluminescent layer, is called the “upper layer”;in general, the bands of the upper layer of electrodes are parallel toand centered on those of the electroluminescent layer, which they atleast partly cover.

Depending on the case, the light emitted by the panel has to passthrough the substrate in order for the images to be displayed to reachthe observer (in the case of “back-emitting” panels), or does not haveto pass through the substrate in order to reach him (in the case of“top-emitting” panels).

In general, the light emitted by the panel must pass through one of theelectrode layers, either the lower layer (in the case of back-emittingpanels) or the upper layer (in the case of top-emitting panels), beforeexiting via the exit face of the panel where it enters the air in thedirection of the observer; the other layer is then generally reflectivein order to recover the light from the cells that is emitted in theopposite direction to that of the observer and to redirect it toward theoutside of the panel via the exit face of the panel.

One of the electrode layers is thus generally transparent, for examplebased on ITO in order to serve as anode, the other then preferably beingmade of metal, which is absorbent or even reflective.

The large index difference between the electroluminescent emitting layerand the air considerably limits the level of light extraction; this isbecause any ray reaching the interfaces between the emitting layer andthe air at an angle of incidence greater than the critical angle ofrefraction (or angle of total reflection) is totally reflected withinthe panel and generally lost.

To limit these losses, documents JP10-223367, WO 01/33598 equivalent toU.S. 2002-0118271, and JP11-354271 teach how to apply, on the exit faceof OLED-type panels, arrays of lenses that are used to extract light:

-   -   either one spherical lens per cell or one cylindrical lens per        row of cells;    -   or many microlenses per cell.

The light extraction systems described in those documents are based onthe refraction of the light at the exit of the cells, more precisely inthe form of the exit face of the panel, which has a suitable curvatureso that the rays emanating from the cells reach this face at an angle ofincidence smaller than the critical angle of refraction so as to passthrough it.

Other documents, such as U.S. Pat. No. 6,091,384 and U.S. Pat. No.6,229,160 propose light extraction systems applied to electroluminescentpanels of the LED or OLED type.

It is an object of the invention in particular to propose anotherextraction solution based no longer on the refraction of rays emanatingfrom the cells, but essentially on their reflection.

The invention relates to an illumination or image-display panelcomprising a one-dimensional or two-dimensional matrix of organicelectroluminescent cells deposited on a substrate and grouped at leastinto rows, characterized in that it comprises, for each cell or group ofcells, an optical light extraction element which itself comprises:

-   -   an entry interface optically coupled with the emissive surface        of said cell or cells of the group or that of the substrate so        as to capture the rays emitted by said cell or cells;    -   an exit interface, the shape of which has a suitable curvature        so that the rays that emanate from this entry interface strike        said exit interface at an angle of incidence smaller than the        critical angle of refraction at this exit interface so as to        pass through it; and    -   optionally, an intermediate reflective surface that has a        suitable curvature so that the rays emanating from the entry        interface that strike said intermediate reflective surface are        sent toward the exit interface so as to have there an angle of        incidence smaller than the critical angle of refraction at this        exit interface in order to pass through it.

In summary, each optical light extraction element of the panelcomprises:

-   -   an entry interface optically coupled with the emissive surface        of the cells or that of the substrate of the panel, so as to        capture the rays emitted by these cells;    -   an exit interface of suitable shape so that the rays emitted by        the cells pass through it; and    -   optionally, a reflective surface that modifies the path of these        rays so as to reduce the angle of incidence at this exit        interface.

In the case of a two-dimensional matrix of cells, these are also groupedin columns; there therefore may be one optical extraction element percolumn.

Preferably, the optical coupling with the cells or with the substrate isprovided by a layer of adhesive having a refractive index comparable tothat of the material of the optical elements.

The material of the optical extraction elements is transparent; this ischosen, for example, from a conventional soda-lime glass of 1.52 index,a polymethyl methacrylate of 1.49 index or a polyethylene glycolterephthalate of 1.57 index; since this material has a refractive indexhigher than that of air and closer to that of the electroluminescentemitting layer, the rays emitted by a cell of the panel, after they havepassed through the entry interface of an extraction element, lie withina solid angle greater than the solid angle in which they would lie afterhaving passed through an interface of identical shape, but with air,which means that this optical element captures a proportion of theradiation emitted by the cells that is greater than which would reachthe air directly via these cells in the absence of the opticalextraction elements; thus, the optical elements increase the level oflight extraction considerably.

According to the invention, the optical element is specially designed sothat almost all of the rays that penetrate the entry interface emergevia the exit interface,

-   -   either by the orientation of these rays being modified,        especially by reflection, so that they have an angle of        incidence smaller than the critical angle of refraction at the        exit interface, especially if this interface is planar and        parallel to the surface of the substrate;    -   or by the shape of the exit interface being adapted, especially        by it being given a convex shape, so that the angle of incidence        of these rays at this interface is smaller than the critical        angle of refraction at this interface;    -   or by using both these means simultaneously.

The use of these means implies that, for each optical extractionelement, the area of its exit interface is greater than that of itsentry interface; this arrangement allows the distance between the edgesof the emissive regions of various cells or pixels to be increased, thisbeing particularly advantageous, especially in the case of active-matrixpanels; this point will be developed later.

Preferably, for each extraction element, the exit interface if there isno reflective surface, or, where appropriate, said reflective surface,does not have a planar surface element; this is because the non planarcurved surfaces are the best suited for obtaining the highest level oflight extraction; if the extraction elements have reflective surfaces,the invention therefore extends to the cases in which the exitinterfaces are planar, in accordance with most of the examples givenbelow.

In a first family of embodiments of extraction elements, the latter forman array of convex microlenses.

The shape of the convex lens of the exit interface is particularly wellsuited for the rays emanating from the entry interface to make, withthis interface, an angle of incidence smaller than if this interfacewere planar and parallel to the substrate of the panel; by reducing theangle of incidence at the interface with the air, the level of lightextraction is very considerably increased.

The area of each microlens is greater than the area of an emittingregion or of a pixel of the panel.

There may be one microlens per cell, in which case each microlens hastwo planes of symmetry, the intersection of which is preferably centeredon a cell, or one microlens per row or per column of cells, in whichcase each microlens has a plane of symmetry that is preferably centeredon a row of cells or a column of cells.

The subject of the invention is an illumination or image-display panelas defined in the claims below, in which the extraction elements have areflective surface; this reflective surface has a suitable shape so thatany ray penetrating the entry interface of an extraction element emergestherefrom via the exit interface of this element.

The shape of the exit interface of each optical extraction element maybe plane or curved; this shape has a suitable curvature so that the raysthat emanate from the entry interface, directly or via one or morereflections on the reflective surface, strike this exit interface at anangle of incidence smaller than the critical angle of refraction at thisexit interface so as to pass through it.

According to this second family of embodiments of extraction elements inwhich the extraction elements have a reflective surface, this surfacepreferably has a suitable shape so that any ray penetrating via theentry interface of an extraction element emerges therefrom via the exitinterface; this condition is expressed as the “edge-ray” principle,referring in particular to Chapter 4, paragraph 2 of the work entitled“High Collection Nonimaging Optics”, by W. T. Welford & R. Winston,Academic Press, Inc. 1989, page 54; preferably, the reflective surfacehas at least one plane of symmetry and each of the two lines ofintersection of this surface with a plane perpendicular to this plane ofsymmetry forms a portion of a parabola, as illustrated in Chapter 4,paragraph 3 of that work, especially in FIG. 4.3, in which, unlike theinvention, such a surface is used as a concentrator; the entry face ofthe concentrator described in that work becomes the exit interface ofthe extraction element according to the invention and the exit face ofthe concentrator becomes the entry interface; as illustrated in Chapter4, paragraph 5 of this same work, each of the two lines of intersectionforms a succession of portions of parabolas; preferably, the position ofthe axis and the focus of the parabola of each line of intersection, andalso the thickness L of the extraction element, are chosen so as tosatisfy the conditions set out in Chapter 4, paragraph 3 of that work,especially on pages 56–57, so as to satisfy the “edge-ray” principle; torecapitulate, each of the two lines of intersection preferably definesthe reflective surface in accordance with what is called a CPC (CompoundParabolic Concentrator), as defined in this same work.

Thanks to the optical extraction elements operating by reflectionaccording to the invention, it is possible to extract a very highproportion of the light emitted by the cells and panels with a highluminous efficiency are obtained.

Other shapes of reflective surfaces may be used without departing fromthe invention, such as conical shapes or paraboloidal shapes in whichthe two lines of intersection mentioned above would have the same axis.

The reflective surface therefore forms a reflector; there may be onereflector per cell, in which case each reflector preferably has twoplanes of symmetry, that are generally perpendicular, the intersectionof which passes through the center of a cell; there may be one reflectorper row or column of cells, in which case each reflector has a singleplane of symmetry centered on a row or on a column.

The invention also applies to cases in which each extraction element hasboth a lens-shaped exit interface and a reflective surface; preferably,this reflective surface is then of the CPC type described above,examples of such elements are given in Chapter 5, paragraph 8 of theabovementioned work.

In a variant, the extraction element also serves to collimate the light;for each extraction element, the exit interface and/or, whereappropriate, the reflective surface then have a suitable shape so thatthe rays exiting the exit interface lie within a solid angle strictlyless than 2π steradians; the shape of the exit interface and/or that ofthe reflective surface are then advantageously suitable for the raysemitted by the panel to be restricted in the direction of a limitedregion of the space especially intended for the observers of the imagesto be displayed; the efficiency of the panel is therefore appreciablyimproved at no further cost.

Preferably, the optical extraction elements of the panel make up asingle part forming an extraction layer.

This part then brings together all the lenses or parabolic reflectors orother light extraction elements; this arrangement is particularlyadvantageous because it is inexpensive both as regards the manufactureof the extraction elements and their assembly on the panel, since allthe extraction elements are suitably positioned on the various cells ofthe panel in a single operation; furthermore, the extraction layer canthen be used for protecting the cells of the panel, especially againstthe action of ambient water and/or oxygen.

If the extraction elements are made of plastic, this single part canthen be produced very inexpensively by conventional plastics conversionmethods, such as compression molding or injection molding.

In general, the matrix of cells comprises an electroluminescent layerplaced between two layers of electrode arrays, one called the “lower”layer on the side facing the substrate and one called the “upper” layeron the other side, each cell being positioned in a region of overlapbetween an electrode of the lower layer and an electrode of the upperlayer; such a panel may include other electrode arrays, especially inthe case of active-matrix panels.

When the electrode or electrodes of the upper layer are transparent orsemitransparent, the panel is then said to be a “top-emitting” panel;the extraction elements are then positioned on top of this upper layer;preferably, the extraction elements, according to the first or secondfamily of embodiments, make up a single part, form an extraction layerand also form an encapsulation layer that is sealed with respect to thesubstrate so as to prevent the penetration of gases, such as oxygen orwater vapor, into the cells and so as to prevent any risk of theelectroluminescent layer being damaged by these gases; the space thusencapsulated may include adsorbent agents or desiccants, capable ofadsorbing these gases.

Preferably, this desiccant is then placed in cavities that are made inthe thickness of the extraction layer, which cavities are open to theinside of the panel in the direction of the electroluminescent layer andare placed between the extraction elements so that the adsorbent agentdoes not impede the passage of the light.

When the electrodes of the lower layer are transparent orsemitransparent, the panel is then said to be a “back-emitting” panel;the extraction elements are then positioned on the opposite face of thesubstrate from that of this lower layer.

Preferably, it is then the extraction layer itself that forms thesubstrate of the panel; as regards the manufacture of the panel, it isthen on the extraction layer, on the entry interface side, that thevarious layers, in particular the layer of electrodes and the layer ofelectroluminescent material, are deposited, which layers constitute thetwo-dimensional matrix of cells of the panel; the extraction layer isthen preferably made of glass.

According to a variant of the invention, the substrate has a fibrousstructure, the fibers of which are suitable and oriented for guiding thelight from one face of said substrate to the other.

The entry interface of each extraction element may cover a group ofcells, especially a row of cells, or for example a column of cells ifthe matrix is two-dimensional; in this case, each extraction elementpreferably has a plane of symmetry centered on this group of cells, rowor column.

Preferably, each cell of this group emits in the same primary color; inother words, each group of cells then corresponds to cells emitting inthe same color.

The entry interface of each extraction element may on the other handcover a single cell; in this case, each extraction element preferablyhas two planes of symmetry, the intersection of which passes through thecenter of this cell; in the case of a panel with a two-dimensionalmatrix of cells, the extraction elements then form a two-dimensionalarray.

In a variant of the invention, the surface density of extractionelements may be greater than the surface density of groups of cells orcells of the panel.

In the case of passive-matrix panels, the electrodes of the arrayspreferably are in the form of parallel conducting bands of constantwidth; preferably, the electroluminescent layer is then divided intoparallel bands emitting different primary colors and arranged in analternating fashion; preferably, each electrode band of the upper layeris then parallel to and centered on a band of the electroluminescentlayer.

However, the invention is particularly advantageous in the case in whichthe substrate of the panel forms an active matrix; this is because, inorder to integrate an active matrix into electroluminescent panels, itis often necessary to limit the area of electroluminescent emissionspecific to each cell; this limitation is no longer a drawback whenlight extraction elements according to the invention are used.

Finally, the invention applies most particularly to image-displaypanels.

The invention will be more clearly understood on reading the descriptionthat follows, given by way of non-limiting example and with reference tothe appended figures in which:

FIGS. 1 and 2 illustrate back-emitting electroluminescent panels, withbarriers and without barriers respectively, before application of alight extraction layer according to the invention;

FIGS. 3 and 4 illustrate top-emitting electroluminescent panels, withbarriers and without barriers respectively, before application of alight extraction layer according to the invention;

FIGS. 5 to 9 illustrate, in side or sectional view and in back or frontview, top-emitting panels without barriers with, in a first family ofembodiments in which the light extraction elements are in the form ofmicrolenses: the generic case in FIG. 5; the case in which agraded-index plate is used as extraction layer in FIG. 6; the case oflongitudinal lenses placed parallel to the rows or to the columns, inFIG. 7 and FIG. 8 respectively; and the case of an extraction layerformed from a two-dimensional matrix of lenses in FIG. 9;

FIG. 11 illustrates, in cross section, a back-emittingelectroluminescent panel without barriers, which is provided with lightextraction elements in the form of reflectors according to a secondfamily of embodiments of light extraction elements;

FIGS. 10 and 12 to 14 illustrate, in side or sectional view and in backor front view, top-emitting panels without barriers and with lightextraction elements in the form of reflectors according to the secondfamily of embodiments: the generic case in FIG. 10; the case oflongitudinal reflectors placed parallel to the rows or columnsrespectively in FIGS. 12 and 13; and the case of an extraction layerformed from a two-dimensional matrix of reflectors in FIG. 14;

FIGS. 15 and 16 illustrate sectional views of back-emittingelectroluminescent panels, the substrates of which are made up fromfibers serving as light guides in order to pass through them, thesebeing provided with reflectors and microlenses respectively; and

FIG. 17 illustrates a top view of an active-matrix electroluminescentpanel before application of a light extraction layer.

To simplify the description and to bring out the differences andadvantages afforded by the invention compared with the prior art,identical reference numerals will be used for the elements that fulfillthe same functions.

The panel according to the invention therefore comprises:

-   -   a two-dimensional matrix of organic electroluminescent cells        deposited on a substrate and grouped together in rows or        columns; and    -   light extraction elements deposited on each cell or row or        column of cells, forming an extraction layer.

The manufacture of the panel, in this case a passive-matrix panel,excluding the extraction layer specific to the invention, will firstlybe described with reference to FIGS. 1 to 4.

Various conventional methods may be used to deposit in succession, on asubstrate, a lower layer of electrodes in the form of an array, anelectroluminescent layer comprising in general alternating bands thatemit different colors, and an upper layer of electrodes in the form ofan array: it is possible to use, for example, photolithography, vacuumdeposition with a shadow mask, spin coating deposition and/or ink-jetprinting.

As was seen above, two types of panel may be distinguished: namelythose, which are the more common, which are back-emitting i.e. the lightpasses through the substrate and therefore through the lower layer ofelectrodes, and those that are top-emitting, i.e. the light passesthrough the upper layer of electrodes.

For each of these types, two types of structure are conventionallypossible, namely a structure with separating barriers between the bandsof the upper layer of electrodes and that of the electroluminescentlayer, and a structure without separating barriers; the latter structureis generally produced by deposition methods using a shadow mask.

The advantage of separating barriers is that they provide betterelectrical isolation between the rows or columns of cells; theirdrawback is that they entail additional cost.

In general, four panel types are therefore encountered, which will nowbe described separately in greater detail.

In the case of a back-emitting panel provided with barriers, FIG. 1shows a substrate 100, formed by a glass plate, on which a transparentlower layer based on ITO (indium tin oxide) is deposited, which is thenetched to form bands of transparent electrodes 101; in the case of anactive matrix, rectangles of transparent electrodes would instead beetched; next, an electrical isolation layer 102 is deposited, leavingspaces or gaps, in this case rectangular in shape, for eachelectroluminescent cell; an array of linear separating barriers 105,that are parallel and oriented perpendicular to the electrodes 101 ofthe lower layer, these being placed between the gaps or spaces in theisolation layer 102; possible methods for producing the barriers aredescribed in document U.S. Pat. No. 5,701,055 (PIONEER); these barriersare made of insulating material, preferably identical to that of theisolation layer 102; deposited between the barriers 105 and within eachgap or space in the isolation layer 102 is the organicelectroluminescent layer forming bands 103, said layer itself beinggenerally structured in the form of several sublayers comprisingespecially an organic hole-injection sublayer, an organicelectroluminescent sublayer proper and an electron-injection sublayer;to deposit these organic sublayers, a vacuum process is used forexample; to obtain alternating bands of different colors, each color isdeposited by masking the inter-barrier regions corresponding to theother colors; next, bands of electrodes 104, which are generally opaqueand preferably reflective, are then deposited on top of theelectroluminescent bands 103; here again, these bands may be structuredin the form of several sublayers, for example a sublayer based onlithium fluoride (LIF) and a sublayer based on aluminum, which providesthe reflective effect; in this way a back-emitting panel provided withbarriers is obtained.

In the case of a back-emitting panel without barriers, FIG. 2 shows asubstrate 100 formed by a glass sheet on which, as previously, bands oftransparent electrodes 101 are deposited; bands 103 of organicelectroluminescent layers, with a structure and composition that areidentical to those described above, are deposited using a mask or a setof masks—one for each color; these bands 103 are parallel, orientedperpendicular to the electrodes 101 of the lower layer and are arrangedso as to cover the rectangular regions of the cells; next, bands ofelectrodes 104 identical to the previous ones are deposited, through themasks having narrower apertures on top of the electroluminescent bands103; in this way, a back-emitting panel without barriers is obtained.

In the case of a top-emitting panel provided with barriers and withreference to FIG. 3, the process used is as in the case alreadydescribed of a back-emitting panel provided with barriers, in which theposition of the transparent electrodes 101, which now belong to theupper layer, and the position of the opaque electrodes 104, which nowbelong to the lower layer, are reversed.

In the case of a top-emitting panel without barriers and with referenceto FIG. 4, the process used is as in the case already described of aback-emitting panel without barriers, in which the position of thetransparent electrodes 101, Which now belong to the upper layer, and theposition of the opaque electrodes 104, which now belong to the lowerlayer, are reversed.

Other conventional processes for obtaining a two-dimensional matrix oforganic electroluminescent cells may be used without departing from theinvention, especially in the case of active matrices.

Two large families of light extraction elements will now be described,by way of example, these being deposited on each cell or on each row orcolumn of cells:

-   -   extraction elements of the microlens 20 type;    -   extraction elements of the parabolic reflector 30 type.

FIG. 5 illustrates extraction elements in the form of microlenses 20deposited on a top-emitting electroluminescent panel without barriers,as described above with reference to FIG. 4; each microlens 20comprises, at each cell, or each row or column of cells, of the panel:

-   -   an entry interface 21 of aperture L_(E) optically coupled with        the emissive surface of this cell or of the cells of this row or        this column, so as to capture the emitted rays emanating from        the electroluminescent layer 103 through the transparent        electrodes 101; and    -   an exit interface 23 of wider aperture L_(S), the shape of which        has a suitable curvature so that the rays that emanate from the        entry interface 21, like that shown by the solid arrow in the        figure, strike this exit interface 23 at an angle of incidence        smaller than the critical angle of refraction at this exit        interface 23 so as to pass through it.

Thanks to such a lenticular element 20, the extraction of the lightemitted by the cells of the panel is considerably improved.

Preferably, the set of extraction elements makes up a single part andforms an extraction layer 200; this extraction layer may be made of atransparent polymer material, which allows it to be formed inexpensivelyby compression molding or injection molding; this extraction layer mayalso be made of glass; this extraction layer may be joined to the panelby adhesive bonding; the layer of intermediate adhesive (not shown) thenserves as means of optical coupling with the panel.

FIG. 6 shows a variant in which the extraction layer 200′ has a uniformthickness and contains graded-index regions 20′, which act in the sameway as the optical extraction elements described above.

FIGS. 7 to 9 present other variants relating to the shape of themicrolenses of the extraction elements:

-   -   FIG. 7B: each extraction element 20 _(L) has a plane of        symmetry, serves as a row of cells of the panel of FIG. 7A and        is centered on a transparent electrode row 101 of this panel;    -   FIG. 8B: each extraction element 20 _(C) has a plane of        symmetry, serves as a column of cells of the panel of FIG. 8A        and is centered on an opaque electrode column 104 of this panel;        and    -   FIG. 9B: each extraction element 20 _(P) has an axis of symmetry        centered on a cell of the panel at the intersection of a        transparent row electrode 101 with an opaque column electrode        104 of the panel of FIG. 9A, and serves essentially for this        cell.

These embodiments in which the extraction elements are in the form ofmicrolenses also apply to back-emitting electroluminescent panelswithout barriers, such as those described above with reference to FIG.2; the extraction layer 200 is then optically coupled with the surfaceof the substrate 100; because of the thickness of the substrate, whichis generally between 0.3 and 1.5 mm and which is greater, or even muchgreater, than the dimension or diameter of the cells or pixels, theamount of light captured by the extraction layer is smaller than in theprevious cases of top-emitting panels; this drawback is avoided by usingthe extraction layer 200 as substrate, preferably then using agraded-index extraction layer 200′ as shown in FIG. 6.

The thickness of the extraction elements in the form of microlenses orof the extraction layer is a compromise between the level of lightextraction, the desired level of concentration or collimation (seebelow), the mechanical strength and the desired level of protection withwhich the panel is desired to be provided.

FIG. 10 describes extraction elements in the form of parabolicreflectors 30 of the “CPC” type described above, deposited on atop-emitting electroluminescent panel without barriers as describedabove with reference to FIG. 4.

Each parabolic reflector 30 comprises, at each cell, or each row orcolumn of cells, of the panel:

-   -   an entry interface 31 of aperture L_(E) optically coupled with        the emissive surface of this cell 10 _(R), 10 _(G), 10 _(B), or        of the cells of this row or of this column, so as to capture the        emitted rays emanating from this or these cells;    -   a reflective surface 32 that has a suitable curvature so that        the rays emanating from the entry interface 31 that strike said        reflective surface are sent toward an exit interface 33 so as to        have there an angle of incidence smaller than the critical angle        of refraction at this exit interface in order to pass through        it,    -   an exit interface 33 of wider aperture L_(S), whose form is        plane in this case.

The rays emanating from the entry interface 31, such as thoserepresented by solid arrows in FIG. 10, strike this exit interface 33 atan angle of incidence lower than the critical angle of refraction atthis exit interface 33, in order to pass through it.

Preferably, the set of extraction elements makes a single piece andforms an extraction layer 300; preferably, this extraction layer is madeof a transparent polymer material and is formed by compression moldingor by injection molding.

Preferably, the reflective surfaces 32 are formed by aluminizing theregions of the surface of this layer that have to be reflective; in avariant, reflection is provided by total reflection.

The optical coupling at the entry interfaces is achieved using a layerof transparent adhesive with an index close to that of the material;however, by coating the entry interfaces 33 of the extraction layer withadhesive, there is a risk of applying adhesive to the reflectivesurfaces 32, which would be particularly deleterious if the reflectionwere to be provided by total reflection; aluminizing the reflectivesurfaces 32 avoids this drawback.

Thanks to such a reflector element 30, the extraction of the lightemitted by the cells of the panel is considerably improved.

FIG. 11 shows a variant in which the extraction layer 300′ comprisingreflectors 30 identical to the previous ones is deposited on aback-emitting electroluminescent panel without barriers as previouslydescribed with reference to FIG. 2; this variant also differs in thatthe density of extraction elements is much higher than previously—thereare in fact twice the number of extraction elements as those of cells orcolumns of cells, or even of cells; this variant also differs in that itincludes an array of reflector elements 28 placed between the substrate100 and the extraction layer 300′, between the entry interfaces 31 ofadjacent extraction elements; this array of reflector elements 28 makesit possible to further improve the extraction efficiency, as indicatedby the path of the light rays shown in the figure.

FIGS. 12 to 14 show other variants relating to the shape of thereflectors of the extraction elements;

-   -   FIG. 12B: each extraction element 30 _(L) has a plane of        symmetry, serves for a row of cells of the panel in FIG. 12A and        is centered on a transparent electrode row 101 of this panel;    -   FIG. 13B: each extraction element 30 _(C) has a plane of        symmetry, serves for a column of cells of the panel of FIG. 13A        and is centered on an opaque electrode column 104 of this panel;        and    -   FIG. 14B: each extraction element 30 _(P) has an axis of        symmetry centered on a cell of the panel at the intersection of        a transparent row electrode 101 and an opaque column electrode        104 of the panel of FIG. 14A and essentially serves for this        cell.

The two large families of light extraction elements that have just beendescribed are, as already seen, applicable to back-emitting extractionpanels, with the drawback already mentioned that the amount of lightcaptured by the extraction layer is then less than in the previous casesof top-emitting panels, because of the thickness of the substrate; oneway of avoiding this drawback is to use a substrate 100″ in the form ofa fiber-based plate, the fibers 106 of which are orthogonal to theprincipal faces of this plate and are suitable for guiding the lightfrom one face of this plate to the other along the shortest possibleoptical path.

FIGS. 15 and 16 show two variants of this embodiment, in the case of anextraction layer 300 formed from reflectors 30 and in the case of anextraction layer 200 formed from microlenses 20 respectively.

The advantage of using extraction layers in the form of a single part200, 300 with this type of substrate 100″ is that they provide very goodsealing and very good protection of the cells of the panel, which wouldbe insufficiently protected from water and oxygen by the substrate alonebecause of its fiber-based structure, which makes it permeable to waterand to oxygen; the organic materials of the electroluminescent layer 103are in fact known to rapidly degrade under the action of water oroxygen.

In general, the extraction layer, when it is made as a single part, mayadvantageously serve as encapsulation layer in order to substantiallyimprove the protection of the cells from ambient oxygen or ambientwater; this advantage is particularly appreciable in the case oftop-emitting panels, whether the extraction elements belong to the firstand/or the second family of embodiments.

The invention has been described with reference to organicelectroluminescent panels without barriers; it also applies to panelsprovided with barriers, such as those in FIGS. 1 and 3 described above;in the case of top-emitting panels, the height of the barriers,generally less than 10 μm, is not an impediment for the application oflight extraction elements to reflectors, given the curvature of thereflectors that makes it possible to remain far from the barriers.

The two large families of light extraction elements have been describedabove with reference to exit interface shapes in the form of microlensesfor the first family and reflective surface shapes in the form ofparabolas for the second, but other geometrical shapes can be used. Bothfor the exit interface of the first family and the reflective surface ofthe second family, it is preferred to use curved surfaces that are moreeffective for extraction, therefore as opposed to surfaces having planeregions.

In a general variant of the invention, the light extraction means aresuitable for also serving to reduce the aperture of the light emissionconoscope of the panel so as to limit it to the area of the space at thefront of the panel, which is specially intended for those observing theimages to be displayed; the geometry of the exit interfaces 23, 33 ofthe extraction elements and/or that of their reflective surfaces 32 areadapted in a manner known per se in order to obtain this concentrationeffect.

It may be seen that, using light extraction means such as thosedescribed above, each extraction element provides an exit aperture L_(S)very much greater than the emission aperture L_(E) of the correspondingcell; the ratio L_(S)/L_(E) is preferably around 4 when there is noconcentration effect and greater than 4 when there is a collimationeffect. Thanks to the invention, the actual area of emission of theelectroluminescent layer in each cell can be very substantially reducedwithout overall losing light flux within the panel; this is because thereduction in actual emission area in each cell is compensated for by theincrease in light extraction level.

The reduction in actual emission area in each cell is particularlyadvantageous in the case of active-matrix panels because, in this typeof panel, a number of electronic components needed to drive the cellsare etched and inserted into the substrate of the panel, at each cell;now, these components may be bulky and result in the actual emissionarea in each cell being limited; this limitation is no longer animpediment when light extraction means according to the invention areused.

FIG. 17 illustrates three adjacent cells 10 _(R), 10 _(G), 10 _(B) of anactive-matrix electroluminescent panel each comprising one electrode ofthe lower layer connected to a memory component 304, and the singleelectrode of the upper layer (not shown) formed here by a solidtransparent or semitransparent conducting layer; although the electrodesof the lower layer have to be separated from one another, a singleelectrode may on the contrary suffice for the upper layer; the emissionarea of each cell is defined here by the region of overlap of theelectrode of the lower layer with the single electrode of the upperlayer, and not, as in the previously described cases of passivematrices, by the intersection between an electrode of the upper layerand an electrode of the lower layer; the area of the electrode of thelower layer of each cell of the active-matrix panel may advantageouslybe chosen so as to correspond to that of the entry interface of theextraction elements according to the invention.

The invention is particularly advantageous if the two-dimensional matrixof cells is produced by deposition methods using a shadow mask or byink-jet printing, especially the electroluminescent layer and the upperlayer of electrodes; this is because, as it is possible thanks to theinvention to reduce the emission area of the cells, the distanceseparating the electrodes, and therefore the width of the patterns ofthe masks used for depositing the various layers or sublayers of thepanel, can be increased; such masks with wider patterns are very mucheasier to position; consequently, the invention is particularlyadvantageous in the case of panels without barriers, for the manufactureof which it is general practice to use shadow masking methods or ink-jetprinting methods.

The invention also applies to the case of electroluminescent panelswhose cells are provided with photoluminescent converter elements, suchas those described for example in document U.S. Pat. No. 5,121,6214; insuch panels, the electroluminescent layers of all the cells emit in thesame color, for example blue; placed in the red and green cells, abovethe electroluminescent layer, is a photoluminescent element emitting inthe red and green respectively, under excitation by the blue; in avariant, an optical filtering layer may be added, especially for bluelight. In the manufacture of the panels of this type, it is thereforeadvantageous to produce the photoluminescent elements on the extractionelements or extraction layer; for this purpose, cavities may be made atthe entry interfaces of the extraction elements and photoluminescentmaterial may be deposited in these cavities; next, as describedpreviously, the extraction elements or the extraction layer are (is)adhesively bonded to the base electroluminescent panel.

1. An illumination or image-display panel comprising a one-dimensionalor two-dimensional matrix of organic electroluminescent cells depositedon a substrate and grouped at least into rows, said matrix of cellscomprising an electroluminescent layer placed between two layers ofelectrode arrays, namely one called the “lower” layer on the same sideas the substrate and one called the “upper” layer of transparent orsemitransparent electrodes on the other side, each cell being positionedin a region of overlap between an electrode of the lower layer and anelectrode of the upper layer, said panel comprising for each cell orgroup of cells, an optical right extraction element which itselfcomprises: an entry interface optically coupled with the emissivesurface of said cell or cells of the group so as to capture the raysemitted by said cell or cells; an exit interface; and an intermediatereflective surface that has a suitable curvature so that the raysemanating from the entry interface that strike said intermediatereflective surface are sent toward the exit interface so as to havethere an angle of incidence smaller than the critical angle ofretraction at this exit interface in order to pass through it, whereinsaid extraction elements are positioned on this upper layer, whereinsaid optical extraction elements of the panel are made up of a singlepiece forming an extraction layer and wherein the extraction layer formsan encapsulation layer.
 2. The panel as claimed in claim 1, wherein saidreflective surface has no plane surface element.
 3. The panel as claimedin claim 2, wherein said reflective surface of each extraction elementhas at least one plane of symmetry and in that each of the two rows ofintersection of this surface with a plane perpendicular to this plane ofsymmetry forms a portion of a parabola or a succession of portions ofparabolas so that the reflective surface forms a CPC (Compound ParabolicConcentrator).
 4. The panel as claimed in claim 3, wherein, when each ofsaid two Fines of intersection form a portion of parabola, the positionof the axis and of the focus of the parabola of each line ofintersection, and also the thickness L of the extraction element, arechosen so that: this focus F lies approximately on the contour thatdefines said entry interface; this axis passes approximately throughthis focus F and approximately through the intersection S of said planeperpendicular to the plane of symmetry with the contour of said exitinterface on the opposite side from this focus with respect to saidplane of symmetry, and if a′ is the distance that separates this focus Ffrom the plane of symmetry and if a is the distance that separates thispoint S from the plane of symmetry: the focal length of the parabola isgiven approximately by f=a′(1+a′/a), and the thickness L of theextraction element is given approximately by L=(a+a′) √{square root over((1−(a′/a)₂)}.
 5. The panel as cl&med in claim 1, wherein, when theentry interface of each extraction element covers a group of cells, eachextraction element has a plane of symmetry centered on this group ofcells.
 6. The panel as claimed in claim 5, wherein each cell of saidgroup emits in the same primary color.
 7. The panel as claimed in claim1, wherein, when the entry interface of each extraction element covers asingle cell, each extraction element has two planes of symmetry, theintersection of which passes through the center of this cell.